Structural analysis of Sr₈Ga₁₆Ge₃₀ clathrate compound

Crystal structural characterization of the Sr₈Ga₁₆Ge₃₀ compound was carried out with the Rietveld refinement method using x-ray powder diffraction data. The structure corresponds to that of the type-I clathrate hydrate, with the Sr atoms filling the "cages" formed by the Ga and Ge atoms. Anisotropic displacement parameters were refined. The limiting figure for rms values of the Sr(1) atoms enclosed in the dodecahedral cages is a sphere, while that for the Sr(2) atoms in the tetrakaidecahedral cages move is an oblate ellipsoid. A comparison of the sizes of the cages in this structure with the ones in other M₈Ga_xGe_46-x (M=K, Ba, or void) structures shows that once the structure is filled (i) there is very little variation in size of the tetrakaidecahedral cages and (ii) the dodecahedral cages expand only slightly (measurably) as a function of the filler size. This is an important result from the thermoelectric point of view because it means that once the structure is full, the cages will not "close over" the guests, limiting their movement and, thus, raising the thermal conductivity. The melting point of Sr₈Ga₁₆Ge₃₀ is approximately 765°C and the apparent optical gap is 0.05 eV.